1. Field of the Invention
The present invention relates to an improvement of the performance of positive electrodes intended for use in alkaline storage batteries such as nickel-cadmium storage battery and nickel-metal hydride storage battery.
2. Description of Related Art
Portable and cordless electronic appliances have been quickly prevailing in recent years. Accordingly, there have been great demands for small-sized, lightweight and high capacity secondary batteries, capable of repeated uses as the driving power source.
The nickel-cadmium storage batteries have a high energy density per unit weight or volume and are excellent in reliability such as cycle life. Therefore, they have found the application in various portable appliances as the power source to date. Conventionally, these nickel-cadmium storage batteries have used the sintered type nickel positive electrodes, but with the advent of non-sintered type nickel positive electrodes likely to materialize a capacity higher than them. Furthermore, nickel-metal hydride storage batteries using a hydrogen-absorbing alloy in the negative electrode and so forth have been developed as well.
In the non-sintered type positive electrodes, however, nickel hydroxide used for the active materials is low in the conductivity, and thus if the positive electrodes are made by using nickel hydroxide singly, the active material near the core material can participate in reactions but not the active material distant therefrom. The weakness has given rise to a problem that the batteries having a high capacity are unfeasible with the non-sintered type positive electrodes. For this, it is necessary to add a conductive material to the positive electrodes, and the Yuasa Journal (No. 65, 28 (1988)) and so forth have proposed the methods for adding the cobalt compounds such as metal cobalt powder and cobalt oxide powder as said conductive material.
The nickel positive electrode is filled with a nickel hydroxide and a conductive material such as cobalt metal, cobalt oxide or cobalt hydroxide and incorporated into an alkaline storage battery, wherein the cobalt compound is dissolved into the electrolyte as the cobalt acid ions and uniformly dispersed on the surfaces of nickel hydroxide. Then, upon initial charge, the dispersed cobalt compound is electrochemically oxidized into cobalt oxyhydroxide, forming a conductive network connecting one active material with another or the active materials with the porous substrate and producing an effect of improving the active material utilization. On the other hand, it has been reported in JPA 3-78965 and so forth that, by using nickel hydroxide coated with cobalt oxyhydroxide or cobalt hydroxide for an active material, cobalt oxyhydroxide which is a conductive material is physically arranged on the surfaces of the nickel hydroxide particles strongly and uniformly, resulting in the battery characteristics more stable than if cobalt hydroxide in the shape of particles are singly used as the conductive material.
Today when there are great demands for low priced batteries in the market, however, it is not preferable to use those cobalt compounds as the conductive material. The cobalt compounds such as metal cobalt powder and cobalt oxide powder are expensive as the material, and as a result, their use leads to the high price of batteries. Furthermore, there is another problem that the processing cost of coating cobalt on the nickel hydroxide active material is high as compared with the cost of merely adding cobalt thereto
In order to find a solution in these problems, it has been proposed in JPA 63-301461, JPA 63-266770 and so forth that metal nickel, carbon powder and so forth which are cheaper than the cobalt compounds should be contained in the positive electrodes as the conductive material or coated on the nickel hydroxide active material to impart the conductivity. Especially, carbon powder as a material is likely to reduce the manufacturing cost to a very low level, even as compared with metal nickel. Graphite powder and so forth has also been tried, for example, by coating the surfaces of graphite with aluminum oxide to improve the cycle characteristics (for example, JPA 9-115521) or attempting to introduce various other improvements.
Metal nickel has been found to have a problem, however, that parts of the compound are electrochemically oxidized into nickel hydroxide in the alkaline electrolyte on account of charging, resulting in the lower conductivity and thus the deterioration of the cycle characteristics.
On the other hand, carbon powder has a great bulk, and if it is used for the conductive material, it reduces the amount of positive electrode active material that can be filled into a given volume, giving rise to the deterioration of the battery capacity. Carbon powder has another problem that it forms the carbonic acid ions on account of oxygen gas generated upon overcharging and produces certain compounds in association with the hydrogen-storing alloy of the negative electrode. Thus, the capacity of the negative electrode is decreased to deteriorate the cycle life. Furthermore, even if aluminum oxide is coated on the surfaces of carbon to inhibit the oxidization, the inhibition is not good enough as evidenced by the resultant utilization short of 60% after 500 cycles.
The present invention is intended to overcome the problems of metal nickel and carbon powder as described above, and an object of the present invention is to provide a non-sintered type positive electrode for alkaline storage battery having the good cycle characteristics by using a metal nitride having a high conductivity and stable presence in an alkaline electrolyte for a long term as the conductivity imparting component of the positive electrode for alkaline storage battery.
Said object can be achieved by using a metal nitride as the conductivity imparting component in the positive electrodes for alkaline storage battery containing the active material comprising nickel hydroxide as the main component.
There can be enumerated three ways of how said metal nitrides are contained in the positive electrodes, for example: a metal nitride conductive material is mixed with the active material comprising nickel hydroxide as the main component; nickel hydroxide powder is coated with a metal nitride; and nickel hydroxide power is coated with a metal nitride to make the main active material, mixed with the metal nitride particles as the conductive material.
The metal nitrides are represented by such a substance as titanium nitride (TiN), zirconium nitride (ZrN) and so forth. These metal nitrides are very high in conductivity but also in chemical stability.
These and other objects and advantages of the present invention will be readily apparent from the following description of the Invention.